March-April 2009

The 2008 Eastern North Pacific Hurricane Season

by Eric S. Blake and Richard J. Pasch

Tropical cyclone activity during the 2008 eastern North Pacific season was below average. Sixteen tropical storms occurred, of which 7 became hurricanes and 2 became major hurricanes. Although the number of tropical storms was near average, the numbers of hurricanes and major hurricanes (Category 3 or stronger on the Saffir-Simpson Hurricane Scale) were below average. Many of the tropical cyclones experienced strong easterly shear, which led to several weak and short-lived storms. In terms of the Accumulated Cyclone Energy (ACE) index, which is defined by the sum of the squares of the maximum wind speed at 6-hour intervals for all of the season’s tropical storms and hurricanes, 2008 had about 75 percent of the long-term median value of ACE. It is worth noting that 10 out of the past 14 hurricane seasons have been below average in the eastern Pacific. Among the significant events of the year were Hurricane Norbert, the strongest storm on record to strike the western Baja California peninsula, and Tropical Storm Alma, which became the only tropical storm on record to make landfall along the Pacific coast of Nicaragua.

Tropical Storm AlmaTropical Storm Alma had an unusual origin for an eastern North Pacific tropical cyclone. Unlike most tropical cyclones in the basin, it appears that Alma formed without the assistance of a tropical wave. Instead, Alma originated from a nearly stationary trough of low pressure that became established over the far eastern portion of the basin in late May. Shower and thunderstorm activity eventually became organized around the low, leading to the formation of a tropical depression early on May 29, centered about 100 miles west-northwest of Cabo Blanco, Costa Rica. This genesis location is farther east than any eastern North Pacific tropical cyclone on record.